1. Field of the Invention
The present invention relates to a geomagnetic sensor, and more particularly, to a geomagnetic sensor which can calibrate an accurate azimuth by measuring the present degree of tilting using an acceleration sensor and then reflecting the degree of tilting in measurement of the azimuth.
2. Description of the Related Art
A geomagnetic sensor is a device for measuring the intensity and direction of geomagnetism that humans cannot sense. Specially, a geomagnetic sensor using a flux gate is called a flux-gate type geomagnetic sensor.
The flux-gate type geomagnetic sensor has a magnetic core made of high-permeability material such as permalloy, and senses the strength and direction of an external magnetic field by applying an excited magnetic field through a drive coil and measuring secondary harmonic components, which is in proportion to the external magnetic field, using the magnetic saturation and non-linear magnetic characteristics of the magnetic core.
This flux-gate type magnetic sensor, which was developed about the end of the 1930's, has a good sensitivity, is economical and small-sized in comparison to other types of geomagnetic sensors. Also, since the flux-gate type magnetic sensor has a low power consumption and a superior long-term stability, it has been widely used for non-military and military purposes such as mineral vein probes, target detection, control of physical attitudes of an artificial satellite, space exploration, etc., together with detection of weak magnetic fields and measurement of absolute directions of the earth. Also, researches for the performance improvement of the flux-gate type magnetic sensor have been continuously propelled.
Specially, with the gradual development of MEMS (Micro Electro Mechanical System) technology, there have been attempts to develop a low power consumption type subminiature flux-gate sensor using the MEMS technology.
FIG. 1 is a block diagram illustrating the construction of a conventional flux-gate type geomagnetic sensor 10. Referring to FIG. 1, the conventional geomagnetic sensor includes a drive signal generating unit 11, a geomagnetism measuring unit 12, a signal processing unit 13 and a controller 14.
The drive signal generating unit 11 serves to generate an electric signal that can drive the geomagnetism measuring unit 12. Generally, such an electric signal may be a pulse wave or an inverted pulse wave.
The geomagnetism measuring unit 12 may be a three-axis flux gate, but it is preferable to use a two-axis flux gate as the geomagnetism measuring unit 12 since the size of the geomagnetic sensor 10 itself should be small enough to be mounted in a small-sized device such as a portable phone. In the case of using the two-axis flux gate, the geomagnetism measuring unit 12 includes two orthogonal flux gates. Also, the respective flux gate includes a square ring type magnetic core, a drive coil and a detection coil wound on the magnetic core. The drive coil serves to receive the electric signal outputted from the drive signal generating unit 11 and excite the magnetic core, and the detection coil serves to detect an electromotive force induced by the magnetism produced through the driving of the drive coil.
The signal processing unit 13 serves to perform a series of processes such as amplification, chopping, etc., of a voltage component, which is induced from the detection coil in proportion to the intensity of an external magnetic field, and then output voltage values through the X-axis and Y-axis flux gates.
An acceleration sensor 15 is implemented by two orthogonal X-axis (i.e., roll axis) and Y-axis (i.e., pitch axis) flux gates such as the geomagnetism measuring unit 12, and serves to calculate a pitch angle and a roll angle. The pitch angle means an angle measured between the geomagnetic sensor 10 and an X-Y plane on which the geomagnetic sensor 10 is placed in the case of rotating the geomagnetic sensor around the Y-axis. The roll angle means an angle measured between the geomagnetic sensor 10 and the X-Y plane in the case of rotating the geomagnetic sensor around the X axis.
The control unit 14 calibrates the azimuth by normalizing the X-axis and Y-axis flux-gate voltage values to values within a predetermined range using predetermined normalization factors and then substituting the normalized values stored in memory 16 in a specified equation. The normalization factors are required in a normalization process for mapping output values of the sensor onto values in the range of +1 to −1, and include a bias (i.e., offset) factor, a scale factor, etc.
The normalization factor can be calculated by recording geomagnetic sensor values measured whenever the geomagnetic sensor takes one revolution, determining the maximum and minimum values, and then substituting the maximum and minimum values in the specified equation. This process of determining the normalization factor by obtaining the maximum and minimum values as rotating the geomagnetic sensor is called a compensation process.
If the geomagnetic sensor tilts during the compensation process as above, however, the output values of the geomagnetic sensor are changed according to the tilting degree of the geomagnetic sensor, and this may cause an erroneous measurement of the maximum and minimum values. In other words, the normalization factors required for the azimuth calibration may also be affected by the tilting of the geomagnetic sensor occurring during the compensation process.
Accordingly, although the maximum, minimum and average values should be measured as the geomagnetic sensor is rotated in a horizontal state in order to properly calibrate the azimuth by calculating the normalization factors, the geomagnetic sensor may be shaken during its rotation. Also, in the case that the geomagnetic sensor is unavoidably installed in a tilting state in a portable phone, it is difficult to keep the geomagnetic sensor in the horizontal state. Accordingly, the maximum and minimum values may be erroneously measured due to the tilting of the geomagnetic sensor, and this may cause an erroneous calculation of the normalization factors and thus cause an error to occur in the finally obtained azimuth.
The measurement error of the azimuth that occurs after the compensation due to the tilting angle is shown in Table 1 below.
TABLE 1Angle tilting duringMeasurement error occurringcompensationafter compensation 0° ≦3° 3° ≦6° 5° ≦8°10°≦12°20°≦20°30°≦30°
As shown in Table 1, as the angle tilting during the compensation becomes wider, the measurement error occurring after the compensation also becomes greater. Accordingly, if the geomagnetic sensor tilts over a predetermined angle, it cannot properly calibrate the azimuth.